The present invention relates to signal separation circuits, and more particularly to an L+R measurement system for separating the L+R component from a composite stereo signal.
When decoding stereo signals in either the broadcast FM or BTSC stereo systems, it is often desired to obtain the monaural L+R component from the composite signal. This may be done to measure the amplitude or other characteristics of this component, or as part of a decoding process. The BTSC system for television defines a main audio L+R channel up to 15 kHz, a pilot signal at the horizontal sweep frequency of 15.734 kHz, and a double sideband, suppressed carrier stereo subchannel symmetrically about a subcarrier frequency which is twice the pilot signal frequency from about 16.470 kHz to 46.47 kHz. The BTSC stereo signal can be written as: EQU Vc(t)=(Vl(t)+Vr(t))+2K(Vl(t)-Vr(t)) Sin (2pi*Fs*t)+Ve
where Vc(t)=BTSC composite stereo signal; Vl(t)=left channel signal=L; Vr(t)=right channel signal=R; K=gain coefficient of the dbx noise reduction system; 2 pi=6.283183 . . . ; Fs=stereo subcarrier frequency; and Ve=everything else such as the pilot, the second language channel and the operations channel (SCA in broadcast FM). The user of a modulation monitor desires to read the amplitude of the stereo sum term L+R. To design lowpass filters for this system requires filter performance several orders of magnitude beyond performance adequate for an FM broadcast stereo system. The guardbands between the main channel, the pilot signal and the subchannel are much narrower, and a dbx noise reduction compressor is placed in the subchannel, increasing the potential for subchannel to main channel crosstalk. The guardbands between the main channel and the pilot signal and between the pilot signal and the subchannel are only 734 Hz each, with the separation between the main channel and the subchannel being 1.468 kHz. Thus, a significant factor in channel separation is the subchannel to main channel crosstalk.
Subchannel to main channel crosstalk occurs when the lower sideband of the subchannel leaks into the main channel due to inadequate low pass filtering of the audio that modulates the subchannel. This crosstalk is nonlinear, i.e., it is highly offensive to the ear because it is not harmonically related to the main channel signals. Additionally when the signal levels are low but the program material contains substantial L-R content, the dbx noise reduction compressor can cause subchannel levels to be 20-30 dB higher than main channel levels. In this situation the main channel has negligible ability to psychoacoustically mask the crosstalk. Further the greatest gain by the dbx noise reduction system is likely to be produced at high frequencies--the very frequencies that appear at the edge of the lower sideband and which are most likely to cause audible crosstalk.
Filters that will pass the sum term and reject the difference term must have such sharp cutoff characteristics that they will ring and overshoot when pulse tested, resulting in erroneous peak readings. High performance filters which are adequate for the BTSC system also are quite complex, one such having as many as 29 poles of filtering overall, are expensive and require great stability.
What is desired is an L+R separation system which separates the L+R component from the composite stereo signal without having stringent filtering requirements.